if-else对字典的理解在python3中不起作用

Question

dlist=['All my loving','All my bros','And all sis']

I would like to create a dictionary such that all words (as keys) are assigned a value which is index of dlist in which the words appear. For example, 'All':{0,1}, 'my':{0,1},'sis'={2} etc.

Somehow this does not work:

dict={}
{w:{num} if w not in dict.keys() else dict[w].add(num) for (num,strn) in enumerate(dlist) for w in strn.split()}

This returns

{'All':{2}, 'my':{2}}

Looks like else statement is being ignored. Any pointers? Thanks

Answer 1

This doesn't work because you are trying to access dict.keys while you are creating dict in a dict comprehension. If this was in a for loop, dict.keys would be updated each element, but the dict comprehensions ensures that the dict is not updated mid-creation to improve speed.

Something like this should work:

myDict = {}
for (num, strn) in enumerate(dlist):
    for w in strn.split():
        if w not in myDict:
            myDict[w] = {num}
        else:
            myDict[w].add(num)